1. Field of the Invention
The invention relates to a method for the surveillance of a moving medium, in particular for the measurement of the filling level in a container or the distance signal of an object to be detected, by use of an electrical sensor, where the electrical sensor is subjected at its input to an adjustable supply-voltage source or to an adjustable supply-current source, and to a device for the performance of this method.
2. Brief Description of the Background of the Invention Including Prior Art
A number of capacity operating or inductively operating electrical sensors are known which usually contain a contactless and non-contacting operating initiator, where the initiator includes an RF oscillator in a blocking oscillator circuit with an active resonant LC circuit or RC oscillator circuit. If an LC initiator is present, the inductivity of the resonant LC circuit is formed as a coil with a radio frequency field aligned with the direction of the coil. If an electrically conducting medium is placed and positioned into the field, then a damping of the oscillator circuit occurs. The oscillation of the oscillator breaks off in case of a sufficiently large damping, depending on the distance of the medium from the coil of the resonant LC circuit, whereby this change of state is transformed into a switching command within an evaluator. If an RC initiator is present, then the sensor electrode is part of the capacitor of the RC oscillator circuit, wherein the capacity of the RC oscillator circuit is capable of changing in case of an approaching of an object with a sufficiently large dielectric constant value at the sensor electrode, whereby the RC oscillator circuit is stimulated to oscillate or whereby the oscillations break off and where the change of the oscillation amplitude can be evaluated in a following envelope detector. Switches with such sensors have the property that the switching distance depends on the material properties and the structure of the body to be detected.
An inductive proximity switch has become known from the German printed patent document DE 40 21 164, where the emitter and the collector of the oscillator transistor are bridged with a capacitor.
Such sensors are in general associated with the disadvantage that no uniform and constant sensitivity is assured based on the temperature dependence of the sensor properties. Thus, a leaving of and a change of the constant switching distance can occur or a stimulation of the oscillator can occur in case of a once performed adjustment of the sensor even before an object or medium is present in the response range. Thus, the switching distance of an approaching object is not constant over the temperature range for operation. Similarly, an application-specific adjustment of the sensor outside the specifications of the sensor can be required, which adjustment can frequently only be accomplished with extreme difficulties.